1. Field of the Invention
This invention relates to the field of nanoparticles and, more specifically, to cellulose-based nanoparticles for drug delivery.
2. Description of Related Art
In the field of controlled drug delivery, the use of polymers to enhance solubility, pharmacodynamics (PK), pharmacodynamics (PD), bioavailability, efficacy, and decrease systemic toxicity is largely defined as polymeric therapeutics1-2. The generalized model of polymer therapeutics is attributed to Ringsdorf3, a model he proposed in 1975, wherein a polymer architecture is modified by solubilizers, drugs, and disease targeting ligands to more effectively dissolve hydrophobic drugs and delivery them in a focused manner.
One rationale for polymer therapeutics is built around the observation that soluble macomolecules with molecular weights (MW) above 30-40 kDa circulate longer in the bloodstream, due to reduced glomular filtration and renal clearance4-6. In 1986, Matsumura and Maeda introduced their observation that tumor vasculature is highly permeable, and that higher MW polymers or particles would selectively accumulate in these tissues as a result.7 It was found that nascent blood vessels in malignant tissues contain gaps through which particles can extravasate8-9, and furthermore, lymphatic drainage in tumors is usually absent: particles can readily enter but not exit the tumor compartment.10-11 Typically, particles smaller than 150-400 nm effectively migrate by the enhanced permeability and retention (EPR) effect, although preferably particles are less than 200 nm in size.12 The discovery of the EPR effect provided significant impetus to develop polymer therapeutics, and in particular gave rise to nanomedicine, where multi-molecule structures comprising liposomes13-15, polymeric micelles,16-18 polymersomes,19-21 and dendrimers22-24 could effectively deliver drug through this passive targeting effect.25 
One important rationale for development of nanoparticles was protection of drugs from metabolism by minimizing interaction of the drug with metabolic processes (improved PD profile).26 In terms of PK however, particles themselves can be opsonized and cleared in the RES (bone marrow, liver, and spleen), reducing the effectiveness of these delivery systems.13, 27 A solution to the RES clearance issue was pegylation: PEG chains conjugated to a polymer or nanoparticle prevent interaction of opsonins with the underlying chemistry through steric hindrance and reduction of hydrophobic and electrostatic interactions, minimizing recognition and clearance of the particle in the RES.28-30 Pegylation became a common element in polymer therapeutics and nanomedicine.13, 31 
In addition to adding stealth properties, particles can be functionalized with targeting ligands to promote the cellular internalization of nanoparticles to specific tissues over-expressing specific receptors, such as folate32, RGD33 and HER234. Although investigators have invested heavily in these studies, limited success with specific targeting has been achieved in drug-delivery products, as tumor accumulation of nanoparticles is largely governed by the EPR effect.35 It has been documented that targeting ligands can increase the blood clearance of the particles,36-37 and that particles internalized though receptor recognition are trapped in the endosome/lysosome organelles and drug is often degraded.38 More successful from a practical point of view has been the incorporation of imaging contrast agents into nanoparticles to enable real-time visualization of the particle (and drug) distribution in the physiology, permitting sensitive non-destructive measurement of biodistribution, and giving rise the field of personalized medicine and theranostics.39-40 For example, superparamagnetic iron oxide nanoparticles (SPIONS) will selectively accumulate in tumors by the EPR effect, and provide MRI contrast.41 Liposomes and polymeric micelles can be loaded with gadolinium compounds, which like SPIONS, provide MRI contrast.42 Polymers can also be labeled with tracers such as 111Indium for microSPECT analysis43, or with dyes such as Cy5.5 for fluorescence imaging.44 
Within the class of polymer micelles, hydrophobic drugs such as paclitaxel (PTX) can be loaded non-covalently in the core of micelle-forming polymers. Notable within this field are the PEG-PLA micelles (Genexol, now in Phase II clinical trials),45 and NK105, a PEG-aspartic acid formulation.45 Both Genexol and NK105 PTX micelle formulations improve upon administration of PTX alone, by reducing formulation toxicity through elimination of the Cremophor-based PTX delivery vehicle which causes hypersensitivity issues in human patients. In comparison to Genexol and NK105, Opaxio (also known as Xyotax and Polyglumex) is a polyglutamic acid polymer conjugated to PTX. Opaxio is in Phase III clinical trials, and to date, represents a promising candidate for approval. When examining the PK data for PTX (free drug), NK105 (micelle), Genexol (micelle), and Opaxio (conjugate), the half lives were 13.3, 10.6, 11.4, and 120 hours respectively, for doses of 210, 150, 300, and 233 mg/m2.16 The PK profile of the non-covalent NK105 and Genexol formulations were nearly identical to that of free PTX, whereas the Opaxio polymer conjugate was the only mode by which PK could be substantially improved. Hydrophobic drugs such as PTX and docetaxel (DTX) will partition from the micelle to plasma proteins including albumin and alpha-1-acid glycoprotein, rapidly depleting the nanoparticle of the drug content.46 Therefore, dramatic improvements to PK and efficacy are likely to be seen only with polymer conjugates.
Currently, the most advanced polymer conjugates (those in clinical trials or approved) are formulated around albumin (Abraxane), HPMA (hydroxypropyl methacrylamide), PEG, polyglutamic acid, with a few other selected examples.1, 47 This is not a long list of polymer compositions despite more than 30 years of research, and the limited selection reflects the cost and complications involved in synthesizing and identifying biocompatible biomaterials. Failure of candidate polymer can occur in a wide variety of modes, including foreign body response,48 toxicity,49 or instability in the biological milieu of hydrolytic enzymes and inflammatory processes.50 Nanoparticle approaches can minimize these complications, as polymer conjugates can be designed to self assemble into structures which present a PEG chemistry (or other suitable shielding entities) to the biological environment, and minimize biological recognition or interaction with the core polymer and drug cargo.27 
Certain classes of polysaccharides are approved as excipients for oral, transcutaneous, and parenteral drug administration,51 but when referenced against the synthetic polymer field, comparatively little work has been done with these biocompatible polysaccharides in the contemporary nanoparticle drug delivery field, and most work has focused on chitosan-based materials.52-55 Cera and co-workers conjugated doxorubicin (DOX) and daunomycin to carboxymethyl cellulose (CMC) and hyaluronic acid (HA), and reported that these compounds were toxic to cells in vitro, but with lower potency compared to the free drug. In addition, the degree of CMC acid group substitution with doxorubicin was low (9%), and this group did not follow up with reports on in vivo efficacy.56 Uglea et al conjugated benzocaine to CMC and oxidized CMC, and tested the effects of these polymers on s.c. sarcoma tumors in rat models, and reported some anti-tumor effect from a single i.p. injection.57 Auzenne et al conjugated paclitaxel (PTX) to HA, and performed in vitro and in vivo efficacy assays.58 Mice were implanted with ovarian carcinoma xenografts in the peritoneal cavity, and were treated with an intraperitoneal injection of 200 mg/kg PTX-HA, a treatment which effectively cured the mice and was well tolerated. There have been no reports regarding the activity of this compound against other solid tumours. Inoue et al reported on a camptothecin analog (DX-8591) conjugated to a carboxymethyldextran via a peptide spacer, which demonstrated strong action against tumours in mice models, and has been tested in 27 patients in a Phase I trial with 1 patient experiencing complete remission, 1 patient experiencing partial remission, and 14 patients experiencing disease stabilization.59-60 In short, while the class of polymer is well known, successful uses of carboxymethyl cellulose for nanoparticle technologies are quite limited.
A substantial fraction of therapeutic small molecules are hydrophobic,61 and rendering these water soluble is, as indicated, a significant rationale behind the development of polymer therapeutics. The taxanes in particular have received special attention, as PTX and DTX command a significant share of the pharmaceutical market, and are currently formulated with Cremophor EL-P/ethanol/saline62 and Tween80/ethanol/saline respectively, each of which causes hypersensitivity reactions, requiring pre-medication of patients being treated with PTX or DTX.63-64 Abraxane (albumin-PTX) and Opaxio (polyglutamic acid-PTX) represent the most advanced PTX conjugates to date, with Abraxane approved for use in the US, and Opaxio in Phase III clinical trials for non-small cell lung cancer47. However, DTX is replacing PTX in clinical applications due to enhanced action,65 and reports on DTX conjugates are indicating that DTX can be more effectively and safely delivered as a polymer conjugate.66-68 